Patterned birefringent elements have been studied for many applications, including displays, optical communications, astronomy, polarization holography, and polarimetry. One way to embody an inhomogeneous birefringence element is using photo-alignment technology combined with liquid crystal materials.
Photo-alignment polymers have been developed for use in liquid crystal displays and other optical elements where it may be desirable to have different orientations of the optical axis at different regions on the sample. Photo-alignment polymers may be exposed (i.e., patterned) using UV lamps or lasers shining through one or more fixed shadow masks, or using holographic intensity interference, to create aligned domains within which there can be predominantly uniform optical axis orientations. Polarization holography can also be used to create alignment profiles with continuous variation of the optical axis orientation to form polarization gratings. This polarization holographic lithography method can be expanded to a general case, where arbitrary linear polarization maps (e.g., spatially-varying polarization patterns) can be produced and recorded from the wavefront of many physical elements. These methods typically involve a small number of exposures with fixed patterns (for example, one or two exposures), without scanning of the illuminating light or shadow mask (if used).
Other methods to fabricate certain patterned retarders with photo-alignment principles may involve scanning in some way, and may be referred to as direct write lithography. For example, a class of waveplates called q-plates (or vortex retarders) can be created using a line- or wedge-shaped aperture and/or lens to create an illumination beam that is rotationally scanned while the recording medium may also be rotationally scanned. This method may use one (or two) dimensions of scanning, and may be limited to spatial patterns that are rotationally symmetric and polarization patterns that are radially uniform. Similarly, a cylindrical lens and a rotating waveplate may be arranged to create a line-shaped beam while the recording medium is linearly scanned in one dimension to produce a polarization grating (PG). This method may use two dimensions of scanning as well, and may be limited to spatial patterns that are one-dimensional.
Other direct write approaches for polarization sensitive media may scan a small polarized spot that has been focused by a lens. In one case, this polarized spot can be scanned across the area of a polarization-sensitive plate using one linear and one rotational stage, and a surface-relief pattern can be recorded by how long each position is illuminated.
In another direct write approach, a polarized spot may be scanned in three dimensions, but all three dimensions may occur in a single element, i.e., the recording medium mount. That is, the spatial scanning and the polarization selection may be inherently coupled. A rectangular aperture may also be arranged immediately before the recording medium to force the illuminating spot to have a step-shaped intensity profile. This configuration may be used to fabricate planar optical waveguides.
Yet another direct write approach provides a fabrication method for computer generated polarization holograms. While this approach may include three-dimensions of computer control, it may also require a square aperture stop to produce a spot at the sample with a step-shaped intensity profile. This can also be used to create an array of discrete exposure squares called “cells” within which the polarization may be uniform, and where overlap of neighboring “cells” may be detrimental. Therefore, stepwise/discrete scanning may be necessary.
In still another direct write approach, a pulsed laser may be used to create a spatially varied form-birefringence in glass. “Micro-waveplates” may thus be formed as discrete spots. A uniform spot with step-shaped intensity may also be required.